The use of liquid carbon dioxide (CO2) as a fracture fluid for stimulating oil and gas containing formations is well known in the art. Utilization of liquid carbon dioxide (LCO2) in the fracture treatment of oil and gas formations has advantages in water sensitive and low pressure formations. First, the use of LCO2 enables a significant reduction in the water load (volume) utilized, which minimizes formation damage caused by the water. Second, the LCO2 energizes the fracture fluid and promotes water flow-back through vaporization and expansion when pressure is removed from the fractured formation.
LCO2 employed in fracture treatments is typically added to a high pressure stream, comprising water and proppant, at the well-head. With the aid of additives such as surfactants and gels, a stable emulsion of LCO2 in water is formed at or near the addition point. The proppant is also dispersed within the water and together with the LCO2 forms, what is known in the art, as the internal phase of the emulsion, with the water forming the external phase. The volume percent of the energizing component (the LCO2), relative to the total liquid phase is commonly termed “the Mitchell quality”, while the volume percent of the total internal phase (the LCO2 and proppant) in the total fluid is commonly termed “the slurry quality”. These qualities are equal when no proppant is present, and the Mitchell quality is lower than the slurry quality when proppant is present. LCO2-based fracturing fluids with a Mitchell quality less than about 53% are commonly referred to as “energized”, while those in the approximate range 53% to 95% are commonly referred to as “emulsions”, although they are still energized by the LCO2. These emulsions have an increased apparent viscosity due to the interaction of the discrete LCO2 dispersed phases with each other. In a similar manner, the presence of proppant as part of the internal phase also contributes to this increase in apparent viscosity. Higher fracturing fluid viscosity can have several advantages such as improving proppant transport within the well bore and fractures, increasing fracture width and reducing fracture face leak-off.
One issue concerning conventional systems for forming high quality fracturing fluids, where proppant is added to the water side, is that as the Mitchell quality (or slurry quality) is increased above 50%, there is a relatively lower flow rate of water available to add the proppant to. This limits the proppant concentration that can ultimately be achieved in the high quality fracturing fluid, and can lead to more fracturing fluid being required to place a fixed amount of proppant through the fracture treatment. Consequently higher treatment costs are incurred with potentially increased damage to the formation through the additional water load.
U.S. Pat. No. 5,515,920 to Luk et al., discloses a method of creating a high proppant concentration, high CO2 content fracturing stream, by simultaneously adding proppant to both the water side, and the LCO2 side. This requires both a conventional proppant blender and a LCO2 proppant blender to add proppant to the respective sides and as can be appreciated, the conventional blender will be somewhat underutilized for the purposes of proppant addition when making high quality fracturing fluids where the flow rate of water is relatively low compared to the flow rate of LCO2.
Another issue with conventional systems for forming high quality fracturing fluids, where proppant is added to the water side, is that as the proppant concentration is varied during a treatment, the internal phase ratio (i.e., slurry quality) in the fluid will tend to vary and consequently, the viscosity of the fracturing fluid will also tend to vary. This can be a significant issue when the proppant concentration is progressively increased from low to high concentrations during the course of a fracture treatment. In order to maintain a substantially constant internal phase ratio (slurry quality), and therefore viscosity, the volume fraction of the LCO2 in the fluid must be dynamically reduced to match the corresponding increase in proppant volume fraction. This requires (i) recognition of this issue on the part of the service provider performing the fracture treatment, and (ii) execution of a control scheme to accurately measure proppant concentration in the water side, with corresponding control of the flow rate of the LCO2 side. As can be appreciated, this can be challenging given the relatively high flow rates of the various feed streams, somewhat rudimentary flow control associated with the high pressure pumpers that regulate the flow of the water and CO2 sides, and the fact that other important parameters such as down-hole pressure are also changing during the course of the treatment.
U.S. Pat. No. 4,627,495 to Harris et al., recognizes this issue and discloses this method of controlling the volume of LCO2 as the volume of proppant material added to the water side varies, but has the aforementioned challenges.
Thus, to overcome the disadvantages in the related art, one of the objects of the present invention is to form a high quality fracturing fluid having a higher concentration of proppant during the course of the fracture treatment than a conventional high quality fracturing fluid, and thus utilizing less water load.
It is another object of the invention, to admix a stream of high pressure liquid CO2 and proppant with a high pressure stream comprising water to form a high quality fracturing fluid which is routed to an underground formation for the fracturing thereof and which has a higher level of proppant concentration during the course of the fracture treatment than a conventional high quality fracturing fluid.
It is a further object of the invention to provide a process for making a high quality fracturing fluid where, proppant is added to the CO2 side only, thereby partitioning the components of the internal phase to be pressurized and flowed through one set of high pressure fracture pump(s), such that the volumetric flow of CO2 is automatically decreased in proportion to the increase in the volumetric flow of proppant, when said high pressure CO2 pump(s) are pumping a constant flow rate, thereby facilitating a relatively constant internal phase ratio (slurry quality) and, therefore, apparent viscosity in the high quality fracturing fluid.
Other objects and aspects of the present invention will become apparent to one skilled in the art upon review of the specification, drawings and claims appended hereto.